Into Thick Air

A couple of months ago, I validated my collapse while climbing a 5200 foot peak by using the alveolar air equation to prove that I was indeed a victim of acute altitude sickness!

Now we are experience the opposite extreme. We are on a dive trip in Placencia, Belize and diving twice daily to a depth of 60-70 feet.

Unlike the atmosphere, where the pressure slowly decreases as you climb, the inverse under water happens much more rapidly. The pressure increases 1 ATM or 760 Torr every 33 feet. Interestingly enough, gravitational acceleration is 33 ft/s^2. Is the linear increase of water pressure of 1 ATM per 33 feet related to gravitational acceleration which is 33 feet/s^2? Or is this just a peculiar coincidence.

We have a theoretical physicist on our trip so I asked him about this numerical peculiarity between dives. His first knee-jerk response was “Just a coincidence, as evidenced by the change of medium one dives in. If you were going down in mercury, which has a higher molecular weight, the pressure would climb much faster.”

“Ah,” I said smugly the way ‘ah’ usually is said, “but we build a lot of our units of measure off of the molecular weight of H2O, therefore, while other mediums may increase pressure with depths at different rates, water could be the base medium from which we originally derived those pressures, and therefore, ergo and so forth, it may still not be a coincidence.”

“Hmm I’ll have to think about that,” he said as he jumped off the boat.

After the dive I approached him, “So, any insights?”

“No. There were turtles.”

Damn turtles. Everyone loves them. But as much as we love to anthropomorphize them, they have brains the size of a pea and probably have more in common with a walnut than a human. So I still don’t know cuz turtles. #ThanksTurtles

So lets table that question for a while, because to truly answer it requires some deep digging into unit factoring and I am in Belize with a rum hangover and unit factoring is not a vacation sport.

Another question to ponder: Is there a limit to how high ones PaO2 can go.

At 66 feet, a common dive depth for 30 minutes or so, pressure goes up to 3 atm or 2280 torr. The air we breathe from a tank is completely dry, and the regulators go in the mouth, circumventing the conchae and mucosa of the nasal cavity that create turbulence and moisture respectively.

if we plug that into our Alveolar Air Equation: PAO2 = ( FiO2 * (PB- PH2O)) – (PaCO2 / RQ)

we get PAO2 = (.21 * (2280 – 0)) – (40/ .8) = 428 torr.

This is similar to what you would see if someone were breathing 70% O2 at sea level. The difference being that, at sea level as FiO2 goes up, there is less and less nitrogen in the mix, and the A-a gradient increases. While diving, it is the pressure that goes up, the gas mixture remains relative, and therefore I can assume the A-a gradient stays constant. Could this be an argument to use barometric chambers on people with severely compromised respiration that isn’t CO based?

Also, people who dive frequently don’t dive on air usually. they dive on Nitrox. Nitrox is essentially any mixture of O2 and N2 where the O2 is not the standard 20.9% as it is in regular air. Nitrox divers tend to dive at a 31% O2 to 69% N2 ratio.

we get PAO2 = (.31 * (2280 – 0)) – (40/ .8) = 656 torr.

This is more similar to breathing 100% O2 at 1 atm. However the A-a gradient would increase slightly due to the change in gas mixture.

The point of diving on Nitrox is less absorbed N2 in the blood and hence less chance of the bends, aka decompression sickness aka caisson disease. This allows longer dive times in the 50-100 foot range of depths because of the reduction in nitrogen. I hear a lot of divers say things like “Well I just feel better when I dive on Nitrox.” No you don’t. You’re not gluten intolerant either. You just get longer bottom times. And when the dive master is limiting your dives to 45 minutes anyway, you’re paying 10 bucks extra per tank for nothing.

However, on the flip side, the divers opens himself up to the potential for oxygen toxicity. There are two kinds of oxygen toxicity: Central Nervous System and Pulmonary.

CNS toxicity occurs when breathing O2 at a partial pressure of greater than 1.6 ata or 1216 torr. Diving on nitrox to a depth beyond 120 feet would put you right on that edge hence the recreational guidelines limit you to 100 feet.

Pulmonary Toxicity is not generally a problem with recreational divers as it is the result of extended exposure to high O2 contents. It can, however, be a problem with long term mechanically ventilated patients on high FiO2.

Ok enough. I’m going diving to look at turtles and forget that I still haven’t answered that other question.

 

 

 

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Decerebrate good times! Come on!

While serving dinner to a few medical professional friends of mine, the concept of “posturing” came up. It first arose in a more metaphorical sense, in the form of political posturing, as in “racists don’t actually give a crap about the flag or people kneeling for the national anthem that they are themselves sitting on the couch drinking a beer through. They are just politically posturing as a way to justify and express their racism.”

But then devolved and digressed into a general criticism of my bad posture.

I naturally stiffened, blamed the over development of my deltoids from years of competitive swimming, invented a disease from which I suffered called “Hyperplastic Swimdrome” and then deflected by criticizing my surgeon friend’s affinity for unkempt hair.

We have spirited conversations. Mostly inspired by spirits. More of the vinic than the chimeric kind.

Fun Fact: Chimerism occurs when one developing zygote absorbs another developing zygote. The end result is one person can actually express two different sets of genetic material. My surgeon friend has one brown and one blue eye. While this can occur for many reasons, chimerism is a suspect. Basically, she ate her own twin. And I will never let her forget it.

After a story predicated on empathy posturing from another medical professional who had just returned from a surgical mission about how “lovely the people of Nigeria were”, I socially postured and pulled out an expensive pinot while making some outwardly glib, but inwardly calculated statement like “Good people: Good wine.”

We might as well have vogued through dinner.

But in the end I learned a few points of medical interest. primarily the differences and implications of decorticate and decerebrate posturing.

Decorticate posturing is position in which a patient has stiff arms bent toward the body with hands and fingers curled into the chest, clenched fists, and legs held out straight. This type of posturing is a sign of damage in the brain stem. And while a very bad sign, not quite as bad as decerbrate posturing.

Decerebrate posturing  is an abnormal body posture in which the arms and legs are held straight out, the toes pointed downward, and the head and neck arched backward. The muscles are tight and rigid. This is a sign of severe damage to the brain.

Another interesting factoid I learned is that, if a patient has their legs crossed, this is a sign of comfort. People who are in pain and actually suffering don’t do this. It is an observation that separates true pain  from drug seeking behavior.

The term “decerebrate” alone means “to remove the brain from.” And what are we doing when we drown ourselves in casks of vino if not that.

So, I rose a glass, and in my best Cool and the Gang voice shouted out, “Decerebrate Good times! Come on!”

*crickets* *tumbleweeds* *a lone wolf howling in the distance*

I don’t care. *I* thought it was funny.

 

 

Into Thin-ish Air

Yesterday Greg, Dave and I pulled into Port Angeles, WA en route to Victoria, BC. We decided to stay a day and do the Hurricane Ridge hike in the Olympic National Park. This hike is a 3 mile in and out with a 900 foot elevation gain starting at about 4350 feet and ending at just about a mile high. We’ll call it 5240 feet because that’s what it says on the map.

The hike felt higher. Like Machu Picccu higher. The last 1/2 mile we met with all the elevation gain and it was steep. Midway I started to breathe heavy. 3/4ths up I was gasping. Another 200 meters I lost the ability to speak. Another 100 meters and I decided I would die on that mountain and hopefully be remembered bravely in a John Krakauer novel.

At the top, I crawled to a bench and collapsed there while a noisy child screeched wildly and was chastised by what must have been his great grandfather who shook his cane at him in annoyance.

“Wow you’re out of shape,” said Greg.

“It’s… the… altitude!”

“That guy made it no problem,” he said gesturing to great grampa.

“He probably… has.. polycythemia,” I stretched.

“Yeah that must be it.”

“Radio my wife,” I said, “I’ll never make it off this mountain and I want to say goodbye.”

“You don’t have a wife. I’m your husband. And you are a drama queen.”

One of the fundamental equations studied in respiratory therapy school is The Alveolar Air Equation. I’ve seen a lot of students struggle with this. So I am not only going to explain it in a clear way, I am also going to vindicate myself and prove to both Greg and his elderly, cane-wielding drag friend Polly Cythemia that I am indeed NOT out of shape and that I was suffering from an acute and debilitating case altitude sickness.

Despite what you may have been told, the actual equation is this:

PAO2 = ( FiO2 * (PB- PH2O)) – (PaCO2 / RQ)

Yes I know the back half of that may be shocking, even disturbing to those of you who learned the PaCO2 * 1.25 method. You can unclench. It’s the same thing.

But let’s start at the very beginning. According to Rogers and Hammerstein, it’s a very good place to start. Working from left to right we have PAO2

Partial Pressure

PAO2 represents the partial pressure of oxygen in the alveoli. P standing for both Partial and Pressure because saying PP always turns grown men into giggling adolescents. stands for Alveoli and O2 standing for Oxygen, because, as you should know, oxygen atoms are unstable alone and stabilize by forming a double covalent bond with another oxygen atom. So each molecule of oxygen in the air is actually made up of two oxygen atoms. Hence O2.

Dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide. For the purposes of respiratory math we can just round the O2 percentage to 21% and and the CO2 down to 0% for inhaled air.

Air, like life, work, school, money and fat-shaming spouses at the tops of mountains, exerts pressure on you. At sea level, that pressure is one atmosphere or 760 torr which we use in this equation because using 1 would be too easy. Torr is a unit of pressure measured in mmHg (millimeters of mercury). The symbol for mercury is Hg because Me would be too easy. In respiratory therapy we often switch between Torr and cm H2O, yet another unit of pressure, because again, being consistent would be too easy.

Anyway, while the units of measure invented by scientists with competing egos may be a convoluted mess, physics is much nicer to you. The pressure of a specific gas in a mixture of gases is exactly proportional to the percentage of that gas in the mixture. Also ‘gases’ is the plural of ‘gas’.  ‘Gasses’ is a verb in the third person present tense. To use both in a sentence: The gassy gascon gasses us with his gases. Also his miasma triggers my asthma. But that’s another story….

Back to partial pressure. For example, if a mixture of gas is exerting 1000 torr, and 10% of that gas is O2, then the partial pressure of that gas is 10% of the total pressure which is 100 torr. Easy as pi.

So real world example, the pressure of air at sea level is 760 Torr. O2 is 21% of that mixture. Therefore the partial pressure of oxygen in air, with 0% humidity, is 21% of 760 torr which is 160 torr. And I hope you can see where I am headed here by now.

Fraction of Inspired Oxygen

Moving to the other side of the equation the first variable we hit is FiO2,  or Fraction of inspired oxygen. In normal air, as we discussed earlier, this is ~.21. As you know, .21 is the same as 21%. But when talking in Fraction of Inspired Oxygen, we notate it as the decimal rather than the percentage. This is because people are petty semantic absolutists who, insecure about their level of knowledge, feign indignant superiority when faced with something they actually know about that you might be doing wrong. Drama ensues. Careers are ruined. The popular RT’s won’t let you sit at their lunch table. I don’t give a personal crap whether you speak in percentages or decimals, just know that in this equation requires the decimal form and that kids are mean.

This is the variable in this equations we as therapists modify to correct for hypoxia. In mechanical ventilation  and with air entrainment devices we can set this with pretty high accuracy anywhere from 21% – 100%.

Atmospheric and Vapor Pressure

PB stands for Barometric Pressure. Or Pressure, Barometric more precisely. This is the total pressure of the gases inhaled. Whether on a vent or an entrainment device or a nasal cannula, at sea level this number is 760 torr and decreases as we get higher in altitude as discussed earlier.

PH2O represents the pressure of the water vapor suspended in the gas mixture. This is relevant to both humidity and temperature. At body temperature (37C) and 100% relative humidity, this is 47 torr. In the ICU we do our best to ensure this constant. In the dry, cold mountain air at 5200 feet, this may have have been zero for me. But in practice, just use 47 torr.

At constant body temperature and 100% humidity, how much this varies with altitude is a question I cannot seem to find the answer to. I assume it would be proportional to the the change in overall PB, but it is not a true gas, it is a liquid in suspension, and therefore assumptions should not be made. Anyone? Anyone? Bueller?

PaCO2/R

This is the partial pressure of carbon dioxide in arterial blood. This is the variable that must be obtained through an arterial line  or through a needle stick usually to the radial artery of the wrist. In the ICUs I have been in most mechanically ventilated patients have an artline in place. These are great because constantly poking holes in people isn’t fun.

Now you may have learned this section of the equation as PaCO2/R with a given R as .8. This is the respiratory quotient. A true measurement of the RQ is the ratio of the volume of CO2 breathed out divided by the volume of O2 taken in. This is, mostly, a function of nutrition. Fats lower the number where carbohydrates raise it.  A legitimate measurement requires indirect calorimetry but for a modern diet, the result will usually be around .8.

However, just using this as a constant instead of an accurate measurement makes an assumption about metabolism that adds a level of error to the calculation. If someone is compromised respiratorily and also has some metabolic derangement affecting the RQ, this could throw things way off. Metabolic derangements that could affect RQ on a cellular level as opposed to a ventilatory level: an interesting thing to look further into.

And to bring you back to familiarity, since the reciprocal of .8 is 1.25, you may often see the back half of this equation written as PaCO2 * 1.25 instead of PaCO2/.8. Same. Exact. thing.

Why, Tho?

How is this information helpful? Where I might just do this because I’m weird and find math fun, others may need some inspiration in the form of clinical significance. That inspiration comes in a measurement know as the A-a gradient. This is the partial pressure of alvolar oxygen minus the partial pressure of arterial oxygen or PAO2 – PaO2. This is a good assessment of the integrity of the alveoli. For a healthy young person, the difference should be between 5 – 10 Torr. After age 20, this gradient increases as the lungs gradually lose their diffusing capacity with age.  A conservative estimate of normal A–a gradient is less than [age in years/4] + 4.

For example for a 40 year old, 40/4 + 4 = 14. the A-a gradient should be right in that ballpark if their lungs are not compromised.

In patients with compromised respiration, even though the alveolar air introduces a bit of uncertainty with its built in assumptions regarding RQ, we can trend this number to see if our therapies are working or not. If this gradient is shrinking, you are winning the fight.

Lastly, ME

At 5200 feet above sea level the air pressure drops to 630 Torr.

Cool Air pressure calculator here: https://www.omnicalculator.com/physics/air-pressure-at-altitude

At a temperature of 50F on top of that mountain the water vapor pressure at, lets assume my nose was functioning properly as the turbulent humidifier that it is, 100% humidity is 13.2

Cool water vapor pressure calculator here:  http://www.endmemo.com/chem/vaporpressurewater.php

Assuming I am healthy we can use a PaCO2 of 40 torr and an RQ of .8. Our FiO2 stays constant at .21. Therefore:

PAO2 = .21(630 – 13.2) – 40/.8 = 80 torr.

Assuming at my age my A-a gradient is 15, then my PaO2 is 65 torr. Normal range for PaO2 is > 80. Mild hypoxia is 70 – 79 torr. Moderate hypoxia is 60 – 69 torr.

65 Torr translates to MODERATE HYPOXIA!

Vindication never felt this good. Or this bad. Kinda both. I guess I’m fine not being able to breathe as long as I am RIGHT.

But then again, why didn’t anyone else struggle?

Hysteresis – Can you handle the pressure?

In pulmonary mechanics we have an hysteresis curve. Note I said *AN* hysteresis curve instead of *A* hysteresis curve. My momma raised me right. Our home was one where infinitives were not something to mindlessly split, Oxford commas were always in play and prepositions were not something to cavalierly end a sentence with.

Much to the dismay of RTs everywhere, brace yourselves, hysteresis is not just a respiratory phenomenon.  Hysteresis is a general term for any system in which the state of the system is dependent upon the history of the system. This dependency creates a a differential when charting state variables, say ‘x’ and ‘y’, of a system cartesianally where the values of y are different depending on whether x is rising or falling.

“Cartesianally”. My momma also was a strong advocate for neologism. And she portmantotally loved portmanteaus.

Hysteresis comes in many forms: magnetic, mechanical, biological; and can be found in almost all disciplines from physics to economics. The specific form of hysteresis we observe in pulmonary measures is mechanical, specifically an elastic form. Essentially, the lung is an imperfect elastic organ and it requires more energy to fill (inspiration), than it does to empty (expiration). Inspiration requires active effort from the diaphragm. Expiration is, or can be, passive. Therefore the volume during a specific pressure is going to be lower during inspiration that it will be during expiration.

The end result *cartesianally* is this:

In respiratory therapy we talk a lot about lung compliance. Compliance is the relationship of a change in volume to a change in pressure (ml/cm H2O) which is represented by the slope of the inspiratory and expiratory curve (whether you like it or not, congratulations, you just dipped your toes into the mythical waters of calculus). The differences in expiratory and inspiratory compliance over the course of one breath cycle are what create this hysteresis effect.

We speak of two forms of compliance in respiratory therapy, dynamic and static. Dynamic compliance is, definitionally, the compliance of the lung at any given moment during the movement of air through the lung, but generally we measure dynamic compliance (Cdyn) at peak inspiratory pressure (PIP), which is the pressure at the end of inhalation.

Cdyn = Vt / (PIP – PEEP)

If, at the end of inspiration, we perform an inspiratory hold, giving the volume of air in the lungs a chance to overcome resistance and equalize. This is the Plateau Pressure or Pplat. We use Pplat to calculate static compliance (Cstat).

Cstat = Vt / (Pplat – PEEP).

Static compliance can be reflective of disease states. Increased Cstat occurs in obstructive diseases such as Emphysema and lowered Cstat occurs in restrictive disease such as fibrosis or pneumonia.

If static compliance stays consistent and dynamic compliance goes down, meaning your PIP increases but your Pplat stays constant, then the practitioner should consider causes that are increasing airway resistance: bronchospasms, retained secretions, a family member standing *unintentionally* on the inspiratory circuit while greedily clutching a will…

Also, this hysteresis curve is one of the many ways in which we as practitioners choose optimal PEEP. PEEP (positive end-expiratory pressure) is a back pressure we can add during ventilation to stint open the alveoli of the lungs and help prevent alveolar collapse and atelectasis on exhalation. The lower inflection point (LIP) of the inspiratory curve, where the change in pressure slows in relation to the volume taken in, representing increased compliance, marks the point at which the alveoli have opened. This is purported to be a perfect place to preset the PEEP preventing atelectasis and atelectrauma. On the graph shown, optimal PEEP would be right around 8 cm H2O.

The pressure at which the alveoli start to become hyperextended and susceptible to volutrauma occurs at the upper inflection point (UIP), where the compliance begins to drop. This curve shows us that we should target a tidal volume where the peak pressures don’t exceed more than 20 cm H2O.

The Hamilton G5 mechanical vent now comes equipped with a Pressure Volume tool that can be used on sedated patients performing an alveolar recruitment maneuver which steps up the PEEP by chosen increments over time monitoring the changes in Cstat and, at the end, recommending PEEP and other settings for the patient.

Most mechanical ventilators, even ones without this tool, will graph this Pressure/Volume hysteresis curve for you real time and it is useful for making a multitude of clinical decisions. If you can handle the pressure. 🙂

Plethysmography!

Plethysmography. That word! Quite possibly the hardest word to pronounce in the English language. Right up there with “thistle” and “chrysanthemum.” One of those dreadful words with legitimate “th’s” in them that thound like mithtakes and remind me that I therioulthy thuffered in thixth grade from a lithp.

Thuper. Theven years of thpeech therapy dithmantled in a thingle paragraph.

But beyond the manner in which it forces me to emotionally decompensate into a bullied preteen, plethysmography is like physics magic! And I’ve discovered that even those who use it daily as part of their PFTs often don’t really understand how mathematically awesome it is underneath.

From here on in I will refer to plethysmography as simply ‘pleth’ because it is just as annoying to type as it is to pronounce.

Pleth measures changes in volume. In respiratory therapy, we use it to calculate the FRC, Functional Residual Capacity, of the lungs. But it can also be used to calculate blood volume changes in the limbs, cerebral blood flow, and a more recent study, an exercise in “duh”, used penile pleth to measure changes in blood volume in the penis showing that, duh, the more homophobic men are, the more aroused they are by gay porn.

https://www.ncbi.nlm.nih.gov/pubmed/8772014

But that’s another story. Never mind. Anyway…. <–Sondheim Quote. Felt appropriate here.

To obtain lung volumes, we put the patient in a sealed box of a known volume that has a pressure transducer in it to measure changes in pressure. The patient is connected to an outside air source and another pressure transducer is placed at the opening of the mouth.

And if you believe in the physics – AND YOU SHOULD! – knowing the starting volume and the pressures of the box and the mouth opening gives you enough information to calculate the lung volume at end tidal expiration (not forced expiration), aka the FRC.

Our story begins with the Ideal Gas Law. It’s good to have ideals. And laws. Gas… not so much.

This law is expressed by the equation:  PV = nrT ,  where P,V and T are Pressure, Volume and Temperature respectively.

n = number of moles of gas. Neither a rodent nor a spy, a mole in chemistry is 6.02×10^23 p/mol.
r = Boltzmann constant = 1.38066×10^23 J/K

Both of these constants have rich, interesting science and history behind them but all you really need to know is that, in a closed system, they are both constant. And hey, if they are always constant why not just combine them into one and call it “k” for “constant” because “constant” starts with…. k? It must be a foreign language thing. I blame the Germans. But just roll with it. K? (It actually has to do with the standard conventions of proportionality mathematics.)

We’re left with: PV = kT

Well to make things even simpler, in our little body box closed system, T also is a constant. Or it becomes constant after a minute or so when the body heat of the person and the ambient temp in the box reaches an equilibrium. Until that happens you just make the patient sit there, trapped, on the edge of claustrophobic panic.

So with T constant, we can roll it right into k and reduce the equation to:

PV = k.

Pressure and Volume are inversely proportional. Meaning that as the pressure rises, the volume falls. And vice versa, if the volume rises, the pressure falls. It’s a simple, linear relationship. This was observed by scientist Robert Boyle’s in 1662 when he published his findings and labeled it ‘Boyle’s law’, stating that the volume of an ideal gas is inversely proportional to its absolute pressure at a constant temperature. They call these factoids of physics “laws” but they are more like “absolute truths”. You can break a law. You can’t break *this*.

So in a system where there is a change in volume

P1*V1 = P2*V2 <– This is everything. Know it. Live it. Love it. Become one with it.

Now back to our box:

The patient is asked to breathe normally from the mouthpiece for a while (because being trapped in a box breathing through a tube makes that so easy) until the end tidal baseline becomes steady and consistent. Just when they get comfortable, on exhale, you occlude the mouth piece. It’s probably best to warn them that this is going to happen to avoid unnecessary flailing and eye bulging.

When the patient tries to breathe against the occluded mouthpiece their thoracic cage expands and contracts and the pressure being measured at the oral opening rises and falls in proportion.

Also, the volume of the box falls and rises and its pressure also varies accordingly.

Here’s where it get’s fun! Or even *more* fun. Cuz, I’m already there.

Starting with the pressure and volume of the box during the occlusion maneuver: We know the initial Pressure – P1. We know the initial Volume – V1. And we measure the pressure after the thoracic volume change on an attempted inhale – P2.

And since P1*V1 = P2*V2, this allows us to solve for V2.

V2 – V1 = the change in volume of the box. Which is – TA DA!!! – the change in volume of the thoracic cage on attempted inhale.

Now to the patient! Like the box, we know the pressure before inhale – P1, and the pressure after inhale – P2. And we now know the change in the Volume of the lung – dV (d is for Delta. Normally a triangle but I don’t know how to make that character on here).

So to solve for the volume of the lung at baseline tidal exhale, or FRC, (V1), we are left with this simple equation.

P1 * V1 = P2 * (V1 + dV)

Now just plug in the known values and, voila, you have the V1, or the FRC of the lungs. With the FRC, you can now extrapolate total lung volume, a critical measure to help diagnose pulmonary diseases!

Wasn’t that fun? I thought it was fun. Fun, fun, fun. Plethysmography is fun!

Now I have to call my speech therapist.

 

First Pretty, Then Fat

Despite the rather suspect title, and although my mirror is a daily reminder of the horrors of gravity and my ever-shortening telemeres, this isn’t a post about body shaming and the steady decline of beauty. Yes I may have gone from “Baby got Back!” to “Baby got Back Pain!” over the past few decades, but when it comes to my body, I have no choice but to dwell *in* it so I try not to dwell *on* it.

“First pretty, then fat” is a process I created for examining chest x-rays. I PRIMP and then I BLIMP. These two acronyms are, respectively, for assessing the quality of an x-ray and then studying the x-ray for anomalies.

First I PRIMP

PRIMP –  for assessing quality
Position – AP, PA. Lateral, Lordotic, Decubitus
Rotation – my favorite sentence “The spinous processes of the thoracic vertebrae should be equidistant from the medial ends of the clavicles.”
Inspiration –  The 5th to 7th anterior ribs should intersect with the diaphragm on the mid-clavicular line.
Margins – Can you see the entire lung field from the costaphrenic angles up to the apices of the lungs.
Penetration – The vertebrae should be just visible behind the heart

Then I BLIMP

BLIMP – Just a mnemonic to remember the major areas to cover
B
ones
L
ungs
I
ssues with tissues – It’s about the tissues. I needed a damn “I” to make it rhyme. 
M
ediastinum
P
leura

One of my clinical rotation shifts I was asked to set up a vent for an incoming patient I knew nothing about so I asked for the details.

“Male, 6’1″, approximately 40, found down for an unknown time, requires lots of suctioning of thick purulent secretions… and oh… the xray they took for ET tube placement showed situs inversus!”

I was excited. I had never seen situs inversus before and I had read about its connection to primary ciliary dyskinesis recently. Again, read too much about zebras – one starts to see zebras.

“Thick purulent secretions? Situs inversus? Was it just dextrocardia or full situs inversus? Was the gastric bubble on the right? was the left hemidiaphragm raised to compensate for the liver? Is there a history of Kartagener syndrome – primary ciliary dyskinesis?”

I annoy people with questions. All of this was unknown. No history. She hadn’t personally seen the x-ray.

So we pulled it up. The computers were slow so I literally had time to primp before it was time to PRIMP. But there it was.

“Position: Portable AP. Rotation… No,” I said, “That is not situs inversus. The gastric bubble is appropriately on the left. And the heart looks odd because the patient is rotated about 30 degrees to the right on the saggital axis.”

“How can you tell?”

Here it was! My big moment. The one I had been rehearsing for! My chance to use my favorite sentence!

“The spinous vertebrae of the equidistant clavicles should be medial to the thoracic… GODDAMMIT!!”

My big moment. And I blew it.

“You mean the spinous processes of the thoracic vertebrae should be equidistant from the medial ends of the clavicles?”, said my understudy Shirley MacLaine, who laughed at my Carol Haney broken ankle and went on to win the Tony for The Pajama Game.

“Who is Carol Haney” you ask?

Exactly.

 

 

Mechanical Ventilation – The Basics!

Mechanical ventilation can be a daunting subject for people to tackle. The sheer volume of information about the subject can put a lot of pressure on a student.

See what I did there? Volume? Pressure? I am _that_ good.

But getting a basic grip on mechanical ventilation can be troubling. That’s trouble and that starts with “T” and that rhymes with “P” and that stands for Proprietary naming conventions.

While all ventilators do basically the same things and have the same set of features, there is no agreed upon industry naming conventions or taxonomies that have been standardized for simplification. Instead, the opposite has happened. Vendors, driven by sales quotas and stockholder greed, fancify their vents with special modal aliases so now we are far more likely to push the wrong button and make our patients explode.

#ThanksCapitalism

We are left with this unnecessary translative layer of abstraction between the basics of mechanical ventilation and the novelty nomenclatures on the knobs causing pneumos.

So to start, ignore the vents themselves and learn the basic modes and functions of mechanical ventilation as a science rather than as a specific machine. Then, once comfortable with that, you can translate that to apply it to a variety of machines later.

Starting with a sedated, paralyzed patient who has no spontaneous breathing we generally have two choices: Volume control or Pressure Control.

Control Issues

‘Control’ comes in two forms: Controlled Mechanical Ventilation (CMV) and Assist-Control Ventilation (ACV). ACV allows patients assisted spontaneous breaths. CMV does not. The only cases I have seen where CMV was used were in high spine injuries where the phrenic nerve had been severed and there was no chance of spontaneous breath, and in late-stage neuromuscular diseases such as ALS or Muscular Dystrophy. In other cases ACV is commonly used, giving the patient a soupçon of respiratory autonomy should they wake from their sedation/paralysis.

In both pressure and volume control ventilation, assisted or not, you set an FiO2 (fraction of inspired oxygen), anywhere from room air (21%) to pure oxygen (100%) depending on the patients needs. You also set the Positive End  Expiratory Pressure (PEEP). This back pressure sets the delta between the alveolar pressure and atmospheric pressure and it helps prevent alveolar collapse, aka atelectasis, at the end of expiration. Generally, all mechanically ventilated patients start at 5 cm H2O. However, setting ‘optimal’ PEEP, the PEEP at which there is best oxygenation without any cardiovascular impediment, is a whole field of study in and of itself. I’ll write about that in later posts. Lastly, you set the breaths per minute (frequency), which again, depending on the patient and disease state, usually starts at anywhere from 10-16 breaths per minute.

Then, for volume control, you set the tidal volume: the volume of air per breath. Usually somewhere around 6-8 ml per kg of ideal body weight landing somewhere between 400-600 ml per breath. then, we titrate the volume down if the pressures are too high.

For pressure control, you set the peak pressure in cm of H2O, usually starting in a safe range, around 20 cm H2O and titrating from there to achieve a desired volume. Obviously the more pressure, the more volume.

So essentially ventilating with these two different control variables achieves the same result but in volume control we have to titrate to manage the pressures and in pressure control we titrate to manage the volumes.

I:E

The I:E ration in mechanical ventilation is the ration between the inspiratory and expiratory times. If you have a frequency set to 15 breaths per minute, then you have 4 seconds per breathing cycle. The percent of that cycle that is inspiratory and the percent that is expiratory is up to the technician. In general, again as always depending on the patient and disease state, we start with a 1:2 ratio. That means, with a 4 second total cycle, 1.33 seconds of inspiration and 2.67 seconds of expiration.

In volume control, on most ventilators, you also choose the I:E ratio directly. The shorter the inspiratory time, the higher the flow will be to achieve the set volume.

In pressure control you adjust the flow rate. The higher the flow, the faster the cycle pressure is reached, the shorter the inspiratory time. So here, flow is what determines the I:E ratio and this must be managed indirectly.

But Wait… There’s More

There’s always more. For patients who are spontaneously breathing there is Synchronized Intermittent MandatoryVentilation (SIMV) or Pressure Support (PS). Different vendors have different forms of ventilation that are closed loop dual-control and adjust the volume and frequency based on the pressures to achieve a constant minute ventilation. For the Hamilton G5 this is called Adaptive Support Ventilation (ASV) and Drager basically the same thing but with a different name because, I dunno, some CEO read Ayn Rand as a teenager and decided empathy was a character flaw.

Mechanical ventilation is a gigantic topic and it’s application varies from patient to patient and disease state to disease state. But to get started, you must understand the basics of pressure and volume controlled ventilation. Hope this helped.